The invention relates to sensor cell structures. More particularly, the invention relates to an oxygen sensor and a combustibles sensor which can function as separate sensors or can function as a gross-oxygen-gross-combustibles sensor for the measurement of combustibles and oxygen in the presence of each other.
Solid state oxygen sensors and combustion sensors consisting of an electrochemical cell which utilizes an oxygen-ion conducting solid electrolyte are known in the art. These sensors operate in two basic modes, one of which is based on a potential measurement while the other is based on a current measurement.
In the potential measurement type of sensor, oxygen activity in an unknown environment is compared to oxygen activity in a known environment according to the Nernst-relationship. The presence of fuel determines the oxygen activity at high temperatures. Therefore, one can, when conditions are correctly chosen, determine the combustibles content of a gas through oxygen activities measurements. However, a low oxygen activity in an inert gas can produce a voltage signal indicating the presence of combustibles when no combustibles are actually present. As a result of this phenomenon, potential measurements require that the gas composition be approximately known to draw conclusions concerning the combustibles content thereof.
The sensor can also function in a current measuring mode which analyzes the oxygen concentration by measuring the current which can be supported by the electrochemically active gas species. In the instance of excess oxygen, oxygen ions are transported through a solid oxide electrolyte from a first electrode to a second electrode. If the access of oxygen is restricted to the electrode from which oxygen enters the electrolyte as an ion, the current becomes indicative of the oxygen ions that are allowed to travel between the electrodes. The current detecting sensor is not an absolute measuring device and must be calibrated with known gas mixtures. The current measuring mode is generally used to analyze gas at a temperature above 800.degree. C. by using a stabilized zirconia electrolyte.
In the presence of excess fuel, most or all oxygen is eliminated at the temperatures at which a current mode sensor is useful because of chemical reaction with the fuel. The electrode which is in contact with combustibles is at a very low oxygen activity while the other electrode is maintained at a high oxygen activity through contact with an oxygen reference, such as air. The cell produces a voltage according to the Nernst-relationship which can drive a current caused by oxygen ion flow in the direction of the lower oxygen activity side. Such a cell operates in a fuel cell mode. Through the restriction of the access of combustibles to the low oxygen side, and through calibration, the current mode can be advantageously utilized to measure excess fuel. At present, the current mode requires higher temperatures than the voltage mode, because of the reduced electrolyte resistance required to enable the development of an adequate current which is limited by the access of combustibles to one electrode.
It is an object of this invention to provide a sensor cell structure which usefully employs the current measurement mode at much reduced temperatures even below 500.degree. C.
It is an additional object of this invention to provide a thin film electrolyte, solid state sensor cell structure which can measure oxygen and combustibles in the presence of each other at such low temperatures that the gases will not react with each other.
It is also an object of this invention to provide a solid state sensor cell mounted on a porous substrate, whereby the sensor cell electrolyte is a thin film layer which effectively seals the porous substrate.